Filters for the removal of solids and other contaminants contained in liquids, whether of the gravity, pressure or vacuum type, are generally comprised of granular filter media of various sizes. (Sometimes the filter media bed has only one filter medium, with all of the particles being of the same composition and substantially the same particle size, but by custom the material of which the bed is made is even in such a case referred to in the plural as the "filter media.") A liquid may be filtered by downward or upward flow through the filter media bed.
In the case of a downward flow filter, the liquid being filtered flows downwardly through the filter media bed and through the media support bed (if any), and exits the filtration tank through openings in an underdrain system. In the case of an upward flow filter, the unfiltered liquid enters the filtration tank through openings in an underdrain system, flows upwardly through the media support bed, if any, and through the media bed itself, with the filtered liquid exiting the filtration tank through collection troughs, or other collection devices, located above the media bed.
Whether a filter is of the downflow or upflow type, cleaning of the filter media (commonly referred to as "backwashing")--either by water alone, by air and water sequentially, or by air and water simultaneously--always involves an upward direction of flow.
The filter media usually comprise layers of granules of various sizes. In the case of a downflow filter the coarsest granules are at the top of the bed and the finest granules, commonly sand or garnet, are at the bottom. In an upflow filter the bed would normally consist of granules of one material, with the coarsest at the bottom and the finest at the top of the media bed.
Most often a media support bed, comprising graded layers of gravel or other suitable material, is provided to serve as a transition between the filter media bed and the underdrain system. The size of the gravel or other material comprising the support bed is larger than the size of the media granules at the interface between the support bed and the filter media bed. The main purpose of the media support bed is to provide a barrier against the possible migration of media particles into the underdrain system, and beyond.
A media support bed would not necessarily be required in a particular filter (1) if the smallest filter media particles in the filter are larger than the openings in the underdrain system that permit the discharge and upward flow of fluid to clean the filter bed, or (2) if the underdrain discharge openings are provided with some type of media retaining device such as, for example, media retaining nozzles or a mesh screen. However, if no gravel support bed is employed and the filter media bed is directly supported on top of the underdrain system, the discharge openings for the exit of washing water or scouring air into the filter media bed (or the openings in the mesh screen, as the case may be) must be extremely small to keep the smallest particles of the filter media out of the underdrain system, and the extremely small size of these openings frequently leads to plugging of the underdrain system. Thus it is generally desirable to provide a support bed of gravel or other suitable material, or some type of prefabricated or precast transition section (usually consisting of a porous ceramic or plastic transition layer), between the underdrain system and the filter media bed itself.
Filters both of the downflow and upflow type have for many years been cleaned by backwashing. During this process, water or a combination of water and air (either sequentially or simultaneous) is passed through the filter bed in an upward direction, which in the case of a downflow filter is opposite to the direction of fluid flow during filtration.
During the filtering process, the underdrain system controls the flow of the liquid that is being filtered so that it is distributed as uniformly as possible over the entire horizontal cross-sectional area of the filter media bed. In a downflow filter, the underdrain system also provides uniform collection of the water after it has passed through the filter bed.
From time to time, cleaning of the filter media becomes necessary because of the increasing resistance to flow caused by the accumulation of suspended solids that have been captured by the filter media, and are attached to filter media particles or are lying in the interstices formed by adjoining filter media particles. During the cleaning of a downflow filter, the underdrain system controls the flow of water (and air as well, when it is used) in the reverse direction from the direction of flow that occurs during the filtering process.
The maximum rate of liquid flow through the filter media bed is as a rule greater during the relatively short periods of time that the filter media bed undergoes backwashing than the rate of flow is during the normal filtration mode. For this reason, the description in this specification of the operation of the present invention is confined to its operation during backwashing and scouring of the filter bed.
When air is used in the scouring of a filter bed, the air bubbles up through the filter and provides a very thorough agitation of the particles in the filter media. The agitation dislodges accumulated dirt and/or gelatinous floc, which can then be removed easily by the liquid backwashing, whether carried out at separate times from, or simultaneously with, the air scouring. This thorough agitation of the filter media is particularly useful for cleaning those filters in which heavy, sticky deposits are formed in the media during the filtering process.
In many of the known underdrain systems for filter beds, gases and liquids flow through common passages to common points of discharge. The fact that in these systems physically very disparate fluids--a liquid and a gas--are simultaneously conveyed, and discharged, through the same channel and opening has in practice often led to very serious problems. Two of these are that (1) because of the conflict between the flow rates of the two fluids, strict upper limits are imposed on the liquid and gas flow rates, and (2) the intermixing of liquid and gas caused by the common passages and discharge openings results in coalescence of the air bubbles into larger bubbles, and in unwanted turbulence.
If such coalescence occurs inside a conduit used for both water and air, the larger bubbles thus formed can act like valves and impede the flow of water through a number of discharge openings, which may produce damage to the system as the water backs up. In some prior art underdrain systems, the intermixing of water and air and the resulting coalescence into larger air bubbles take place outside the conduits for the liquid and gas, in the gravel support bed above the underdrain, before the two fluids enter the filter bed itself. The resulting turbulence can produce an undesirable movement and expansion of the gravel support bed.
The goal of backwashing and scouring is to loosen and agitate the filter media itself as much as feasible. In fact, in the case of a filter bed that has more than one filter media layer, the materials comprising the bed are preferably selected so that their size and specific gravity will mean that the entire bed will fluidize at the same backwash flow rate. In contrast to this, any loosening and expansion of the support bed directly under the filter media bed is not desired, because it may disrupt the physical integrity of the filter bed lying above it. Thus it is important that intermixing of the streams of water and air and the resulting turbulence be avoided, to the extent possible, at all times prior to the introduction of the two fluids into the filter media bed itself.
Still another reason for avoiding intermixing of the washing liquid and scouring air as long as possible, preferably until the two fluids are actually introduced into the filter media bed, is that the likelihood of maintaining a high degree of uniformity in the distribution of the water and air within the filter media bed is greater if the intermixing first occurs in the media bed itself. Lack of uniformity of distribution of backwash water and scouring air within the filter bed can seriously impair the filtering action of the filter because various portions of the filter bed may remain contaminated even after backwash. In addition, non-uniform distribution can disrupt the bed. Furthermore, non-uniform distribution of wash water within the filter bed tends to result in the formation of "mud balls," which are balls of contaminant that form in small portions of the bed through which minimum backwash water flows. Non-uniform distribution may also cause "sand boils," and even shifting of the media and/or piling up of the media particles in some portions of the bed. It is then often necessary to remove the filter media and place a completely fresh filter media bed.
Backwashing a filter bed with water and scouring with air, carried out either sequentially or simultaneously, have been known for a very long time. The use of water and air in this way is referred to, for example, in U.S. Pat. No. 801,810 issued to Parmelee on Oct. 10, 1905, as being already well known at that time. Over the intervening decades, a very large number of underdrain systems for cleaning beds of filter media in this way have been devised. However, there has been no recognition in these systems of the desirability of avoiding intermixing of the washing water and scouring air at all times prior to introduction into the filter media bed itself. In fact, most of these systems have permitted, or even deliberately brought about, mixing of the water stream and air bubbles at some point before they are introduced into the filter media.
The advantages of an integrally constructed, completely self-contained module that can be assembled side-by-side with other modules to form an underdrain system for a filter bed have also been long recognized. (Typical of such prior devices is the device disclosed in U.S. Pat. No. 2,378,239 issued to Myron on Jun. 12, 1945.) However, the only other underdrain system known to the applicant that entirely precludes the intermixing of water and air within an integrally constructed underdrain module (U.S. Pat. No. 4,995,990 issued to Weston on Feb. 26, 1991) does not suggest any of the following definite advantages that are provided by applicant's invention
(1) An underdrain system the top surface of which is substantially flat, covering substantially the entire surface area of the underdrain system, thereby providing virtually unlimited flexibility for the spacing of water and air discharge openings.
(2) Horizontal separation of air and water openings.
(3) The avoidance, or even the minimizing, of intermixing of water and air within the support bed of gravel or other material before the rising streams of water and air bubbles reach the filter media bed itself.